The present invention relates generally to optical scanners and, more particularly, to an apparatus for assembling components of color optical scanners.
Optical scanners are used to produce machine-readable data which is representative of a scanned object, e.g. a page of printed text. Optical scanners employ line-focus systems to image scanned objects.
In a line-focus system, a light beam from an illuminated line object is imaged by a lens on a linear photosensor array which is positioned remotely from the line object. The linear photosensor array is a single dimension array of photoelements which correspond to small area locations on the line object. These small area locations on the line object are commonly referred to as "picture elements" or "pixels." In response to light from its corresponding pixel location on the line object, each photosensor pixel element in the linear photosensor array (sometimes referred to simply as "pixels") produces a data signal which is representative of the light intensity which it experiences. All of the photoelement data signals are received and processed by an appropriate data processing system.
In a color optical scanner, a plurality of spectrally separated imaging beams (typically red, green and blue beams) must be projected onto photosensor arrays. Some color optical scanners employ beam splitter devices for spectrally separating an imaging light beam into color component beams. These separate color component beams are projected onto separate linear photosensor arrays.
FIG. 1 is a schematic diagram showing a cut away view of a lens assembly, a trichromatic beamsplitter device, and a three-line photosensor array of an optical scanner. As shown in FIG. 1, an input polychromatic imaging beam 12 is focused by a lens 14 which is held by a lens holder 16 capable of focusing the lens very precisely on the detector surface of a three-line CCD array 18. Input beam 12 impinges upon the optical surfaces of an optical separator assembly 10 and is separated into three predetermined colors to form three optical beams having predetermined spectral ranges that are carefully selected by dichroic layers 21, 23, 25, 27, 29, 31 placed on optical surfaces of the optical separator 10.
As shown on FIG. 1, a lower dichroic layer device or filter member 20 of separator 10 is disposed such that the angle of incidence of the optical axis of input beam 12 is approximately 22.5.degree.. Input beam 12 is split by filter member 20 into three spatially and spectrally separated beams that are transmitted to an upper dichroic layer device or filter member 22 which is also disposed at approximately 22.5.degree. to the optical axes of each of the spatially and spectrally separated optical beams. The upper dichroic layer device 22 is normally constructed in the same manner as the first dichroic layer device 20 but with the order of the dichroic layers reversed so that the three separate optical beams that are transmitted from the second dichroic layer device have equal optical path lengths to a predetermined image plane. For example, device 20 may have equally separated red, green, and blue reflective layers 21, 23, and 25 and device 22 may have red, green, and blue reflective layers 27, 29 and 31 which are separated by the same distance as the layers of device 20.
The detector device 18 is disposed on an image plane that is substantially normal to the optical axes of the separated input beams which are focused thereon. Each of the three spatially and spectrally separated optical beams is focused on a separate line detector array 33, 35, 37 (which are shown in cross-section in FIG. 1 and thus appear as points) on detector 18 so that a line scan of, for example, a document results in each of the colors from the line scan being detected simultaneously on the detector surface of detector 18 as a result of the equal optical path lengths of each of the individual spectrally separated beams.
The dichroic layer devices 20 and 22 are precisely held in the positions illustrated by a mounting device 24 that includes arm structures 28 and 30 that extend between two side portions. The mounting device 24 is open in the central portions to allow light to be transmitted to the optical component 10 and subsequently to detector 18. Detector 18 is also precisely located in the mounting device 24 by way of interface surfaces 32 and 34. Signals derived from detector 18 are fed directly to circuit board 36 via connectors 38 and 40 that comprise a plurality of connectors.
The construction and operation of color optical scanners employing beam splitter assemblies and photosensor arrays such as described above with reference to FIG. 1 are fully disclosed in the following United States patents which are all hereby specifically incorporated by reference for all that is disclosed therein: U.S. Pat. No. 4,870,268 of Vincent et al. for COLOR COMBINER AND SEPARATOR AND IMPLEMENTATIONS; U.S. Pat. No. 4,926,041 of Boyd for OPTICAL SCANNER; U.S. Pat. No. 5,019,703 of Boyd et al. for OPTICAL SCANNER WITH MIRROR MOUNTED OCCLUDING APERTURE OR FILTER; U.S. Pat. No. 5,032,004 of Steinle for BEAM SPLITTER APPARATUS WITH ADJUSTABLE IMAGE FOCUS AND REGISTRATION; U.S. Pat. No. 5,044,727 of Steinle for BEAM SPLITTER/COMBINER APPARATUS; and U.S. Pat. No. 5,040,872 of Steinle for BEAM SPLITTER/COMBINER WITH PATH LENGTH COMPENSATOR.
In assembling optical scanners of the type which employ beam splitter devices (filter members) such as described above, it is imperative that the filter members 20, 22 be accurately positioned with respect to each other and with respect to the associated photosensor unit 18. Misalignment of these components can produce multiple types of error in the output of an optical scanner in which the components are mounted. Such error includes image registration error (centering, spacing, and alignment error), focus symmetry error, and color registration error.
The alignment task is complex due to the multiple axes about which the filter members must be accurately located in order to be properly positioned with respect to an associated photosensor array. The task is further complicated by the fact that it must be accomplished relatively quickly if the associated scanner is to be a mass-produced product.